1. Field of the Invention
The present invention relates generally to methods of forming resist patterns, and more specifically, to an improved method of forming resist patterns which allows formation of highly sensitive and good patterns using ultraviolet ray of a short wave length, or high energy beams such as electron beam and X ray. The present invention further relates to a method of manufacturing photomasks using such a method.
2. Description of the Background Art
DESIRE (Diffusion Enhanced Silylating Resist) process is dry development resist process presented by Coopman and Roland in 1986. FIGS. 10(a)-(e) illustrate the process flow of DESIRE process.
Referring to FIG. 10(a), a film 2 is formed on a substrate 1. A dry development resist 3 is formed on film 2. Dry development resist 3 is, for example, a PLASMASK.RTM. resist manufactured by JAPAN SYNTHETIC RUBBER Co. Ltd. The composition of PLASMASK.RTM. is not known in detail, but the resist contains novolac resin shown in FIG. 11(a) and quinondiazide shown in FIG. 11(b) form the principle constituents.
Referring back to FIG. 10(a), using a mask 4, resist 3 is selectively irradiated with ultraviolet light 5 of a wavelength 248-436 nm. Referring to FIG. 10(b), the selective irradiation of the ultraviolet light 5 divides resist 3 into the exposed part 3a and the non-exposed part 3b. At the exposed part 3a, quinondiazide decomposes in accordance with the reaction equation shown in FIG. 13.
Referring to FIG. 10(c), while heating substrate 1 to 120.degree.-200.degree. C., the mixed gas of a hexamethyldisilazane (HMDS) gas and a nitrogen gas is blown onto substrate 1. This treatment is called silylating process. The mixed gas of the HMDS gas and nitrogen gas is produced by bubbling nitrogen into liquid HMDS.
This silylating process causes HMDS to selectively diffuse into the exposed part 3a and react with the novolac resin in resist 3, thus forming a silylated layer 3c. Silylated layer 3c contains silylating compounds as shown in FIG. 12.
The selectability is provided for the following reason. At the exposed part 3a of resist 3, the quinondiazide decomposes into an indene carboxylic acid derivative. By the effect of the indene carboxylic acid, HMDS tends to diffuse to the side of the exposed part 3a rather than the non-exposed part 3b in which quinondiazide remains intact.
Referring to FIG. 10(d), with substrate 1 being exposed to O.sub.2 plasma, silylated layer 3c turns into silicon oxide (SiO.sub.2) 6. The silicon oxide 6 is highly resistant to O.sub.2 plasma, and, therefore, etching does not proceed immediately under the part in which the silicon oxide 6 is formed. Meanwhile as shown in FIG. 10(a), etching does proceed in the part in which HMDS has not diffused. This is because the resist in this part consists of only the materials such as C, H, N and O which evaporate by oxidation.
This selective etching leaves a pattern only in the exposed part and a resist pattern of negative type is formed.
As described above, quinondiazide in the resist should be turned into carboxylic acid during exposure in the conventional DESIRE process using the PLASMASK as a resist.
In order to manufacture finer patterns, light of a shorter wavelength (a wavelength not more than 250 nm), or high energy beam such as electron beam and X ray should be used.
However, using high energy beam such as these, the reaction shown in FIG. 13 in which quinondiazide in the resist changes into carboxylic acid does not proceed efficiently. Also with the use of such high energy beam, side reaction such as cross linking of novolac resin takes place, impeding pattern formation. Also, the sensitivity is extremely low in this case.